How much spandex is too much? How much is not enough? What size should I get to allow for stretching and shrinking without getting the baggy butt?

The current jeans I have stretched enough in the waist/hips that I need to wear a belt or I'm constantly pulling them up (they have 98% cotton and 2% spandex). This weekend I got a pair of jeans (different brand than my current ones) with a 33 waist, which are a bit tight and slightly squishing me when I sit down. I also tried on the 32 waist but they were definitely too tight, although I was able to do them up. These are 99% cotton (I believe) and 1% spandex.

Were the "slightly squishing" jeans the right choice, or should I have gone with the "barely done up" ones?